Tear-Off Interface

Suppose you have a COM server with object A which implements I1 and object B which implements I2. But then there is a change in requirements, and A should be changed to support I1 and I2.

In the usual case, a COM object supports a number of interfaces through multiple inheritance. Another option is to make I2 a tear-off interface of object A. If QueryInterface is called on an instance of A requesting I2, it will create and instance of B and return its interface pointer.

Tear-off interfaces can be cached or non-cached, which is kind of strange terminology. Cached means a singleton, non-cached means a separate instance for each request.

Tear-off interfaces are only practical using ATL.

BEGIN_COM_MAP(CCoreObject)
    COM_INTERFACE_ENTRY(ICoreObject)
    COM_INTERFACE_ENTRY(IDispatch)
    COM_INTERFACE_ENTRY_TEAR_OFF(IID_IUseThisRarely, CAuxilliary)
END_COM_MAP()

The use cases for tear-offs are when interfaces are optional, or transient.

Tear-Off Interface

References